Project Summary: HIV and substance abuse represent two epidemics that disproportionately affect young men who have sex with men (YMSM). The majority of HIV transmissions among YMSM occur in the context of serious relationships, largely due to higher rates of condomless anal sex (CAS) with serious compared to casual partners and lack of PrEP uptake in relationships. Unfortunately, research on risk factors for HIV and substance abuse has largely focused on individuals with limited attention to dyads. In addition to stress experienced by all couples, coupled YMSM experience unique stress related to their sexual minority status (e.g., discrimination, stigma). This added stress may lead to problems in relationship functioning (e.g., dissatisfaction, conflict, intimate partner violence), which may then contribute to engagement in health risk behaviors, such as CAS and substance use. Delineating these processes is critically important to inform dyadic prevention strategies to optimize relationship functioning and reduce health disparities among YMSM. The proposed study will utilize data from a subsample of a large, racially/ethnically diverse cohort of YMSM who are currently being recruited for a NIDA-funded U01. The cohort is being built by having YMSM recruit their serious relationship partners into the study, providing the unique opportunity to examine dyadic influences on HIV risk behavior and substance use among YMSM and their partners. This proposal will also provide the applicant with postdoctoral training in HIV and substance use research, statistical methods for analyzing dyadic data, and grantsmanship, all of which are necessary to launch his career as an independent scientist. The applicant will learn how to implement the Actor-Partner Interdependence Model, which statistically accounts for the nonindependence of data in dyads by simultaneously estimating actor effects (a person?s influence on his own behavior) and partner effects (how much a person is influenced by his partner). This model will allow the applicant to examine the influences of individual- and partner-level stressors on risk behaviors. Further, dyadic data provide the opportunity to examine the extent to which concordance/discordance of stress between partners contributes to each partner?s HIV risk behavior and substance use above and beyond the individual actor and partner effects. In sum, the goals of the proposed study are to examine: (1) actor and partner effects of stress on HIV risk behavior and substance use; (2) the associations between concordance/discordance of stress between partners and HIV risk behavior and substance use; and (3) problems in relationship functioning as mediators of these associations. This proposal represents formative research on dyadic influences on health risk behavior among YMSM, which is a necessary first step in order to develop evidence-based prevention and intervention programs that target the context in which HIV risk transmission is highest (romantic dyads). Findings from this study will inform our understanding of health disparities by disentangling individual- and partner-level influences on HIV risk behavior and substance use in a high-risk population (diverse YMSM).